1. Field of the Invention
The present invention relates to beamforming techniques for phased array antennas and, more particularly, to methods and apparatus for shaping antenna beam patterns.
2. Description of the Related Art
Phased array antennas are used in a wide variety of applications to transmit and receive directed beams of electromagnetic energy. The shape and direction of the antenna beam produced by such antennas are determined by the relative phases and amplitudes of the signals at the individual antenna elements that constitute the phased array. For example, where hardware permits the relative phases of the antenna elements to be adjusted during operation, the main lobe of the antenna radiation pattern can be steered over a range of different directions to transmit a signal in a selected direction or to receive a signal arriving from a particular direction.
It is customary to employ non-uniform illumination, sometimes referred to as amplitude tapering, over array apertures in order to alter radiation patterns from what they are with uniform illumination. Often, such amplitude tapering is used to reduce sidelobes in the resultant radiation patterns. With uniform illumination, although the boresight transmitted power is maximized, approximately twenty percent of the radiated power is transmitted into the sidelobes. With the use of tapered illumination functions, the power in the sidelobes can be reduced to a few percent of the total, with well over ninety percent being confined to the main beam. To date, no methods other than some form of tapering have been in use for these purposes.
The resultant tradeoff for reduction in sidelobe levels by means of tapering is an unavoidable broadening of the main beam relative to the uniform-illumination case, which reduces the array gain and, therefore, the transmitted and received power levels. Power reductions by one-half are typical. Accordingly, where sidelobe reduction is sought, it would be desirable to augment or replace conventional tapering techniques with techniques capable of achieving greater sidelobe reduction without a corresponding further broadening of the main beam and the attendant reduction in peak antenna gain. More generally, it would be desirable to have greater control over the shaping of the antenna radiation pattern produced by a phase-array antenna than that afforded by conventional techniques.